The clinical phenotype of patients with severe hemophilia varies widely despite similarly low factor levels (<1% of normal activity). Even in individuals with the same causative genetic mutation and factor levels, there are significant differences in joint bleed frequencies, age of onset of joint bleeding, and requirements for factor Vlll (fVIII). The biological determinants that drive these different bleeding patterns in patients with severe hemophilia are largely unknown. This lack of knowledge considerably hampers the clinician's ability to individualize treatment decisions In patients with severe hemophilia. Development of a method that distinguished severe hemophilia patients with mild and severe clinical phenotypes would have multiple clinical benefits. Examples Include the potential to adjust the start or cessation of prophylactic therapy based on an individualized assessment of bleeding risk, and the identification of patient subgroups that would be most likely to benefit from experimental therapies. Novel concepts and assays are proposed that will allow the stratification of severe hemophilia A patients according to bleeding risk. The central hypothesis is that platelet phenotype contributes to bleeding phenotype in patients with hemophilia A, and hemostatic assays that incorporate platelet function will allow prospective determination of bleeding phenotype in patients with severe hemophilia A. Two complementary pathways of investigation are proposed; the first focused on platelet procoagulant activity and the regulation of platelet-fVIII interactions, the second on the assessment of hemostatic plug formation in a dynamic fluid environment. Aim 1 will test the hypothesis that individual differences in platelet procoagulant activity contribute to the variability in bleeding phenotype observed in patients with severe hemophilia A and investigate the mechanisms regulating the binding of procoagulant proteins to activated platelets. Aim 2 will test the hypothesis that differences in bleeding phenotype can be prospectively identified by assessment of hemostatic potential in a novel endothelialized microfluidics model. A systematic series of experiments examine the impact of variations in platelet phenotype and coagulation factor concentrations on hemostatic plug formation In PRP and whole blood in this novel system.